The present invention relates to a process for making porous activated carbon and more specifically to a two-cycle carbonization/activation process for making porous activated carbon materials. The invention relates also to porous activated carbon made according to the inventive process.
Energy storage devices such as electric—also called electrochemical—double layer capacitors (EDLCs), a.k.a. supercapacitors or ultracapacitors may be used in many applications where a discrete power pulse is required. Such applications range from cell phones to electric/hybrid vehicles. An important characteristic of an energy storage device is the energy density that it can provide. The energy density of the device, which typically comprises one or more carbon-based electrode(s) separated by a porous separator and/or an organic or inorganic electrolyte, is largely determined by the properties of the carbon-based electrodes and, thus, by the properties of the carbon material used to form the electrodes.
Indeed, the performance of an energy storage device comprising carbon-based electrodes is largely determined by the physical and chemical properties of the carbon. Physical properties include surface area, pore size and pore size distribution, and pore structure, which includes such features as pore shape and interconnectivity. Chemical properties refer mainly to bulk and surface impurities, the latter relating particularly to the type and degree of surface functionalization.
Carbon electrodes suitable for incorporation into EDLCs are known. High performance carbon materials, which form the basis of such electrodes, can be made from natural and/or synthetic carbon precursors. For example, activated carbon can be made by initially heating a natural or synthetic carbon precursor in an inert environment at a temperature sufficient to carbonize the precursor. During the carbonization step, the carbon precursor is reduced or otherwise converted to elemental carbon.
Examples of suitable natural carbon precursors include coals, nut shells, woods, and biomass. Examples of suitable synthetic carbon precursors, which generally yield higher purity carbon material than natural carbon precursors, include polymers such as phenolic resins, poly(vinyl alcohol) (PVA), polyacrylonitrile (PAN), etc.
Following the process of carbonization, the carbonized material can be activated. During the activation step, the elemental carbon produced during the carbonization step is processed to increase its porosity and/or internal surface area. An activation process can comprise physical activation or chemical activation.
Physical activation is performed by exposing the carbonized material to steam or carbon dioxide (CO2) at an elevated temperature, typically about 800-1000° C. Activation can also be carried out by using an activating agent other than steam or CO2. Chemical activating agents such as phosphoric acid (H3PO4) or zinc chloride (ZnCl2) can be combined with the carbonized material and then heated at a temperature ranging from about 500-900° C. In addition to phosphoric acid and zinc chloride, chemical activating agents may also include KOH, K2CO3, KCl, NaOH, Na2CO3, NaCl, AlCl3, MgCl2 and/or P2O5, etc.
As an alternative to performing the chemical activation on carbonized material (i.e., post-carbonization), one or more chemical activating agents can be combined with a carbon precursor in conjunction with a curing step prior to carbonization. In this context, curing typically comprises mixing a carbon precursor with a solution of a chemical activating agent and then heating the mixture.
By curing is meant a heating cycle that at least partially cross-links or polymerizes a carbon precursor to form a viscous or solid material. A cured carbon precursor that optionally comprises a chemical activating agent incorporated throughout its structure can be carbonized and activated. As used herein, a “heating cycle” comprises a heat-up step, a hold step, and a cool-down step, and the temperature associated with a heating cycle is the temperature to which a sample is heated during the hold step.
During a step of curing with a chemical activating agent, the carbon precursor and the chemical activating agent can be in the physical form of solid, solid powder, or solution before they are combined. If a solution is used, it is preferably an aqueous solution and the concentration can range from about 10-90 wt %. The carbon precursor and the chemical activating agent can be combined in any suitable ratio. The specific value of a suitable ratio depends on the physical form of the carbon precursor and the chemical activating agent and the concentration if one or both are in the form of solution. A ratio of carbon precursor to chemical activating agent on the basis of dry material weight can range from about 1:10 to 10:1. For example, the ratio can be about 1:1, 1:2, 1:3, 1:4, 1:5, 5:1, 4:1, 3:1, 2:1 or 1:1.
The curing step can comprise, for example, heating a carbon precursor/activating agent mixture at a temperature in the range of about 100-300° C. for a period of about 1-48 hours. During the heat-up, hold, and cool-down, the mixture is preferably maintained in a reducing or inert environment. One or more reducing gases (e.g., H2, H2/N2 mixtures, CO) and/or one or more inert gases (e.g., N2, He, Ar) can be used.
In embodiments where a chemical activating agent is used, it can be advantageous to homogeneously distribute the chemical activating agent throughout the carbon precursor at a molecular level prior to curing. In such a process, a chemical activating agent in the form of an aqueous solution can be combined with the carbon precursor. This molecular level mixing of the chemical activating agent can produce a homogeneous activated carbon that comprises a uniform distribution of physical characteristics (pore size, pore size distribution, and pore structure etc.).
As an alternative to combining an aqueous solution of a chemical activating agent with a carbon precursor, the chemical activating agent can be mixed with the carbon precursor in solid form.
Following carbonization/activation, the activated carbon product can be washed to remove both the activating agent and any chemical species derived from reactions involving the activating agent, dried and optionally ground to produce material comprising a substantially homogeneous distribution of nanoscale (and/or mesoscale) pores. The washing comprises washing the activated carbon material first with de-ionized water, then an aqueous acid solution, and then de-ionized water.
Activated carbon produced by this method offers significantly higher energy storage capacity in EDLCs compared to major commercial carbons. In addition to its use in energy storage devices, such activated carbon can be used as a catalyst support or as media for adsorption/filtration.
Whether a chemical activating agent is combined with a carbon precursor in solid form or using an aqueous solution of the chemical activating agent, the cured mixture is conventionally carbonized and activated in a single heating cycle. This so called “one-cycle” process is simple and convenient. However, aspects of such a “one-cycle” carbonization/activation process may limit large-scale production of activated carbon material due to economic considerations.
Particularly in embodiments where sodium or potassium salts or bases are used as the chemical activating agent, a large volume of gas can be generated by various chemical reactions that occur at intermediate temperatures during the carbonization/activation heating cycle. The large gas volume can cause foaming of the intermediate product, resulting in a volume expansion of a factor as high as 30-40. This gas production and the concomitant foaming effectively limit the amount of starting material that can be processed in a furnace of a given volume.
When using a chemical activating agent comprising a sodium or potassium salt or base, an additional consideration is the possibility that elemental sodium or potassium can be produced as a by-product of reactions between organic molecules (and/or organic functional groups on carbon) and the activating agent. Metallic sodium and metallic potassium are each very reactive and can explode when exposed to air or moisture. Because these alkaline metals can vaporize and re-deposit in the furnace during the elevated processing temperatures associated with carbonization/activation, the furnace should be built corrosion-resistant and configured to ensure safe operation. This will further increase equipment cost and capital investment.
When taken together, these two factors may limit the utilization of furnace capacity and capital investment. On the one hand, out-gassing during carbonization/activation suggests that larger volume furnaces would be useful in order to accommodate the foamed carbon precursor. On the other hand, the formation of alkaline metals such as sodium or potassium during carbonization/activation suggests that these (larger) furnaces should be fitted with additional features to properly address corrosion and hazard concerns, which adversely affects cost.
In view of the foregoing, it would be an advantage to provide a process for producing activated carbon that enables a more efficient utilization of capital investment while maintaining the attributes of the resulting carbon material.
These and other aspects and advantages of the invention can be accomplished by dividing the thermal processing associated with carbonization/activation into two consecutive heating cycles. According to such a “two-cycle” process, a first cycle comprises heating a mixture of a carbon precursor and a chemical additive at a first (intermediate) temperature, followed by a second cycle in which the material derived from the first cycle is heated at a second (elevated) carbonization/activation temperature. In one aspect, the mixture heated in the first cycle comprises a cured mixture.
During the first cycle, essentially all of the foaming is complete but sodium and/or potassium, if used, is not converted to metallic form in significant amount and does not volatize significantly. Thus, the first cycle can be carried out in a large, relatively low-cost furnace. During the second cycle, although there may be volatilization of sodium or potassium, there is no substantial volume expansion of the carbon precursor. Thus, a smaller, specially-equipped furnace can be used during the second cycle.
This two-cycle process allows efficient utilization of capital equipment, which can translate into significant savings in production cost. It has been demonstrated that the EDLC performance of the resulting carbon material is essentially the same as that of carbon produced in the one-cycle process.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows and the claims.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.